In the normal human heart, the sinus node, generally located near the junction of the superior vena cava and the right atrium, constitutes the primary natural pacemaker initiating rhythmic electrical excitation of the heart chambers. The cardiac impulse arising from the sinus node is transmitted to the two atrial chambers, causing a depolarization known as a P-wave and the resulting atrial chamber contractions. The excitation pulse is further transmitted to and through the ventricles via the atrioventricular (A-V) node and a ventricular conduction system causing a depolarization known as an R-wave and the resulting ventricular chamber contractions.
Disruption of this natural pacemaking and conduction system as a result of aging or disease can be successfully treated by artificial cardiac pacing using implantable cardiac stimulation devices, including pacemakers and implantable defibrillators, which deliver rhythmic electrical pulses or anti-arrhythmia therapies to the heart at a desired energy and rate. A cardiac stimulation device is electrically coupled to the heart by one or more leads possessing one or more electrodes in contact with the heart muscle tissue (myocardium). One or more heart chambers may be electrically stimulated depending on the location and severity of the conduction disorder.
A stimulation pulse delivered to the myocardium must be of sufficient energy to depolarize the tissue, thereby causing a contraction, a condition commonly known as “capture.” In early pacemakers, a fixed, high-energy pacing pulse was delivered to ensure capture. While this approach is straightforward, it quickly depletes battery energy and can result in patient discomfort due to extraneous stimulation of surrounding skeletal muscle tissue.
“Threshold” is defined as the lowest stimulation pulse energy at which capture occurs. By stimulating the heart chambers at or just above threshold, comfortable and effective cardiac stimulation is provided without unnecessary depletion of battery energy. Threshold, however, is extremely variable from patient-to-patient due to variations in electrode systems used, electrode positioning, physiological and anatomical variations of the heart itself, and so on. Furthermore, threshold will vary over time within a patient as, for example, fibrotic encapsulation of the electrode occurs during the first few weeks after surgery. Fluctuations may even occur over the course of a day or with changes in medical therapy or disease state.
Hence, techniques for monitoring the cardiac activity following delivery of a stimulation pulse have been incorporated in modern pacemakers in order to verify that capture has indeed occurred. If a loss of capture is detected by such “capture-verification” algorithms, a threshold test is performed by the cardiac pacing device in order to re-determine the threshold and automatically adjust the stimulating pulse energy. This approach, called “automatic capture”, improves the cardiac stimulation device performance in at least two ways: 1) by verifying that the stimulation pulse delivered to the patient's heart has been effective, and 2) greatly increasing the device's battery longevity by conserving the battery charge used to generate stimulation pulses.
Commonly implemented techniques for verifying that capture has occurred involve monitoring the internal electrocardiogram (IEGM) signals received on the implanted cardiac electrodes. When a stimulation pulse is delivered to the heart, the IEGM signals that are manifest concurrent with depolarization of the myocardium are examined. When capture occurs, an “evoked response” may be detected, which is seen as the intracardiac P-wave or R-wave on the IEGM that indicates contraction of the respective cardiac tissue. Through sampling and signal processing algorithms, the presence of an evoked response following a stimulation pulse is determined. For example, if a stimulation pulse is applied to the ventricle, an R-wave sensed by ventricular sensing circuits of the pacemaker immediately following application of the ventricular stimulation pulse evidences capture of the ventricles.
If no evoked response is detected, typically a high-energy back-up stimulation pulse is delivered to the heart within a short period of time in order to prevent asystole. An automatic threshold test is next invoked in order to re-determine the minimum pulse energy required to capture the heart. An exemplary automatic threshold determination procedure is performed by first increasing the stimulation pulse output level to a relatively high predetermined testing level at which capture is certain to occur. Thereafter, the output level is progressively decremented until capture is lost. The stimulation pulse energy is then set to a level safely above the lowest output level at which capture was attained. Thus, reliable capture verification is of utmost importance in proper determination of the threshold.
Conventional cardiac stimulation devices include single-chamber, or bi-chamber pacemakers or implantable defibrillators. A single-chamber device is used to deliver stimulation to only one heart chamber, typically the right atrium or the right ventricle. A bi-chamber stimulation device is used to stimulate both an atrial and ventricular chamber, for example the right atrium and the right ventricle. It has become apparent in clinical practice that the timing interval between atrial stimulation and ventricular stimulation, known as the AV interval or AV delay, may be important in achieving the desired benefit of bi-chamber pacing. Hence, capture verification in each chamber is important in maintaining the desired atrial-ventricular synchrony.
Mounting clinical evidence now supports the evolution of cardiac stimulating devices capable of stimulating both the left and right heart chambers, e.g., the left and right atrium or the left and right ventricle, or even three or all four heart chambers. Therapeutic applications indicated for bi-chamber (left and right heart chamber) stimulation or multi-chamber stimulation include stabilization of arrhythmias or re-synchronization of heart chamber contractions in patients suffering from congestive heart failure. The precise synchronization of the left and right heart chamber depolarizations is expected to be important in achieving the desired hemodynamic or anti-arrhythmic benefit. Thus, verifying capture in each chamber being stimulated would be essential in maintaining the desired stimulation benefit.
However, in order to achieve bi-chamber or multi-chamber stimulation in a clinical setting, conventional pacemakers have sometimes been used in conjunction with an adapter or a special bifurcated lead so that electrodes may be positioned in both the left and right heart chambers with electrical communication via only one lead connection to the same output channel of the stimulation device.
A four chamber pacing system has been proposed in which unipolar right and left atrial leads are connected via a bifurcated bipolar adapter to the atrial port of a bipolar dual chamber pacemaker. Likewise, unipolar right and left ventricular leads are connected via a bifurcated bipolar adapter to the ventricular channel. The left chamber leads are connected to the anode terminals and the right chamber leads are connected to the cathode terminals of the dual chamber device. In this way, simultaneous bi-atrial or simultaneous bi-ventricular pacing is achieved via bipolar stimulation but with several limitations.
One limitation is that simultaneous stimulation of left and right chambers, as required when two leads are coupled together by one adapter, or by internal hardwiring, is not always desirable. First, such a configuration is sub-optimal in terms of energy delivery because the right chamber lead acts as an additional load during left chamber stimulation and the left chamber lead acts as an additional load during right chamber stimulation.
Second, when inter-atrial or inter-ventricular conduction is intact, stimulation in one chamber may be conducted naturally to depolarize the second chamber. A stimulation pulse delivered in one chamber, using the minimum energy required to depolarize that chamber, often is conducted to the opposing chamber, thereby depolarizing both chambers. In this case, stimulation of both chambers would be wasteful of battery energy.
Precise control of the depolarization sequence and timing may be necessary in order to provide the anti-arrhythmic or hemodynamic support desired. Multi-chamber stimulation systems have been proposed that allow independent stimulation in each chamber, in some cases related to a coupling interval based on sensed or paced events in other chambers.
In order to ensure and maintain a desired depolarization sequence, performing capture verification in each chamber being stimulated is essential. One proposed method of performing capture verification during multisite cardiac pacing verifies capture in one area of the heart by detecting a conducted depolarization in another area of the heart that is electrically continuous with the stimulated area. The limitation of such a method is that it relies on the natural conduction of the depolarization within the cardiac tissue.
In bi-chamber or multi-chamber stimulation, the inter-chamber conduction may not be intact, or stimulation of both chambers may be preferred at a prescribed interval rather than waiting for a naturally conducted depolarization to travel from one chamber to the opposing chamber. Immediate detection of the local evoked response in the chamber being stimulated would be necessary in these situations. As a result, the proposed method would not be appropriate in all patients.
There remains an unmet need for a bi-chamber or multi-chamber cardiac stimulation device that allows independent stimulation and sensing in both right and left chambers of the heart and further provides reliable capture verification in each chamber. It would thus be desirable to provide a system and method for bi-chamber or multi-chamber stimulation with capture verification and automatic threshold determination made possible in each chamber independently.